The present invention relates to a nuclear power plant of water cooling type and, more particularly, to a chemical decontamination method and a chemical decontamination system by which radioactive nuclides are chemically removed from metallic material surfaces of primary cooling system components and pipes and a system including the component and the pipes which are contaminated with radioactive nuclides.
As conventional technologies in connection with chemical decontamination, Japanese Patent publication No.3-10919 discloses a method in which components of a nuclear power plant made of metals are chemically decontaminated using permanganic acid as an oxidizing agent and dicarboxylic acid as a reducing agent. As methods of decomposing the above-mentioned organic acids, PCT/JP97/510784 discloses a method of decomposing the acid into carbon dioxide and water using an iron complex and ultraviolet rays. According to this method, since hydrogen peroxide of the oxidizing agent and the organic acid react by using the iron complex as a catalyst to produce carbon dioxide and water, the organic acid can be prevented from becoming waste products.
Although oxalic acid is used as the above organic acid, oxalic acid has a strong solvency for iron. Accordingly, when the decontaminating solution is allowed to flow through a system made of carbon steel which corrodes easily corroded compared to stainless steel, a large amount of iron ions are dissolved from the carbon steel to increase an amount of produced waste products, or the oxalic acid is precipitated in the form of iron oxalates. Therefore, a sufficient effect cannot be obtained in decontamination using oxalic acid of a system having low corrosion resistant materials such as carbon steel.
In order to apply the method to the system containing the low corrosion resistant materials, it is considered that hydrazine is added to oxalic acid in order to adjust so as to increase the pH of the decontaminating agent. However, since hydrazine is trapped in a cation exchange resin column (hereinafter, referred to as a cation resin column), the load of the cation resin column is increased when the decontaminating solution is allowed to directly flow into the cation resin column. Therefore, the amount of hydrazine exceeds an exchanging capacity of the cation resin column to cause hydrazine to flow out. As a result, the amount of hydrazine flowing out is increased as the load of metallic ions increases to excessively increase the pH of the decontaminating agent and accordingly to decrease the decontaminating effect. In order to avoid this problem, it is necessary to control the concentration of the hydrazine appropriately. The control means preferably decomposes into nitrogen and water. Although hydrazine can be decomposed by irradiating ultraviolet rays onto the hydrazine using a UV column (ultraviolet ray irradiation apparatus), the oxalic acid as well as the hydrazine is decomposed. It is difficult to selectively decompose only the hydrazine, and it is insufficient to reduce the load of the cation resin column because the ratio of decomposing hydrazine is low to produce ammonia. SFEA, ┌Actes de la Conférence Internationale Proceedings of the International Conference┘, Apr. 24-27, 1994, Nice-France page203-210 “A FULL SYSTEM DECONTAMINATION OF THE OSKARSHAMN 1 BWR” by Johan Lejon and Ása Hermansson.